In Python, efficiently determine if two lists are shifted copies of one another

Question

What's the most efficient (in time) way of checking if two relatively short (about 3-8 elements) lists are shifted copies of one another? And if so, determine and return the offset?

Here is the example code and output I'd like:

>>> def is_shifted_copy(list_one, list_two):
>>>     # TODO
>>>
>>> is_shifted_copy([1, 2, 3], [1, 2, 3])
0
>>> is_shifted_copy([1, 2, 3], [3, 1, 2])
1
>>> is_shifted_copy([1, 2, 3], [2, 3, 1])
2
>>> is_shifted_copy([1, 2, 3], [3, 2, 1])
None
>>> is_shifted_copy([1, 2, 3], [1])
None
>>> is_shifted_copy([1, 1, 2], [2, 1, 1])
1

Lists may have duplicate entries. If more than one offset is valid, return any offset.

Answer 1

here's a simple iterator version that does the job in 2n iterations (n being the length of list)

import itertools

def is_shifted_copy(list1, list2):

    if len(list1) != len(list2):
        return False

    iterator = iter(list2)

    for i, item in enumerate(itertools.chain(list1, list1)):
        try:
            if item == iterator.next():
                continue
            else:
                iterator = iter(list2)
        except StopIteration:
            return i - len(list2)

    else:
        return False


print is_shifted_copy([1, 2, 3], [1, 2, 3]) #0
print is_shifted_copy([1, 2, 3], [3, 1, 2]) #2
print is_shifted_copy([1, 2, 3], [3, 2, 1]) #False
print is_shifted_copy([1, 2, 3], [2, 3, 1]) #1
print is_shifted_copy([1, 1, 2], [2, 1, 1]) #2
print is_shifted_copy([1, 2, 3], [1]) #False
print is_shifted_copy([1, 2, 1], [2, 1, 1]) #1
print is_shifted_copy([1, 1, 1], [1, 1, 1]) #0

and from your specification,

shouldn't is_shifted_copy([1, 1, 2], [2, 1, 1]) return 2?